Field
Embodiments disclosed herein generally relate to devices, methods and systems for measuring tissue perfusion.
Description of the Related Art
Reconstructive surgeries, such as flap surgery, consist of the transplantation of healthy tissue, such as a skin graft or flap, from a donor site to a wounded recipient area affected by loss of tissue. The loss of tissue may be related to a specific trauma, such as trauma due to burn, laceration or cancer removal. The transplanted tissue usually comprises skin, underlying adipose tissue, or muscle, but it can also consist of composite tissues (skin and fat, skin and fat and muscle, etc.) or other organs. When possible, the tissue is transplanted from a nearby area without disconnecting the vascular network. However, in certain cases, the tissue must be transplanted from a different area of the body (also referred to as a free flap), and the vascular network is reconnected to the existing blood vessels of the recipient area.
After detachment from the donor site, whether transplanted from a nearby area or different area of the body, it is crucial to re-establish blood perfusion throughout the transplanted tissue in a timely manner and with high accuracy to guarantee a successful long-term outcome. If the tissue is not properly perfused in a timely fashion, the tissue will die through a process known as necrosis. A number of techniques have been developed to determine whether the tissue has been properly perfused, with varying levels of success.
During the reconstructive procedure, surgeons can use a Doppler ultrasound pencil probe to assess if blood flow is re-established in the vessels underneath the transplanted skin. Although of some value, this method only allows coarse assessment of blood flow in major vessels by producing audible feedback which is indicative of blood flow to the surgeon. Specifically, ultrasound techniques can grossly show that blood flow is being received by the area, but they do not provide information regarding the direction of blood flow or the oxygenation of the blood received by the transplanted tissue. Doppler ultrasound is sensitive to blood flow, primarily in arteries and arterioles. Doppler ultrasound relies on the technique and the interpretation of the surgeon of the whooshing sound produced by the Doppler device in response to detected blood flow, which is highly subjective and sensitive to technique. Oftentimes this leads to a binary interpretation (“flow/no flow”) that does not represent the accurate status of tissue oxygenation, and might lead to inaccurate clinical decisions. Moreover, the technique gives no measure of perfusion at the periphery of the transplanted tissue, as it is supplied by much smaller capillaries which are not detectable by use of the Doppler device.
Contrast medium imaging using fluorescent dye is an effective methodology to infer information about circulation in large and medium-sized vessels, but it does not provide reliable information on microcirculation in the transplanted tissue. In addition, it is inherently qualitative and its applicability is limited to the operating room where internal tissues are exposed to direct illumination of the imaging system.
Laser Doppler Flowmetry (LDF) is another technique which has been used to determine blood perfusion. In LDF, a beam of laser light is delivered to a volume of tissue. Blood cells in the volume of tissue which are struck by laser light will partly reflect it, whereupon the light undergoes a Doppler shift. The light in the volume of tissue will be a mixture of unshifted and Doppler-shifted components, the magnitude and frequency distribution of the latter being related to the number and velocity of moving blood cells within the volume of tissue. Similar to contrast medium imaging, LDF can provide imaging of blood perfusion only in superficial skin layers, i.e., tenths of microns below the skin surface.
Near Infrared Spectroscopy (NIRS) has been shown to be useful in plastic surgery and transcranial oxygenation detection. NIRS uses near infrared (NIR) radiation to penetrate the underlying tissue where specific frequencies will be absorbed or back-scattered primarily based on the oxygenation state of hemoglobin. However, current NIRS devices and methods determine oxygenation at a single point or region without mapping or providing oxygenation data based on depth at multiple points in real time.
Thus, there is a need in the art for better visualization of the blood flow and the oxygenation state of the blood, and to track changes over time (including after a patient has been discharged home following surgery).